1. Field of the Invention
The present invention relates to an R-T-B system rare earth permanent magnet (wherein R represents one or more rare earth elements (providing that the rare earth elements include Y), and T represents one or more transition metal elements essentially comprising Fe, or Fe and Co), and especially relates to an R-T-B system rare earth permanent magnet with high magnetizability.
2. Description of the Related Art
Among rare earth permanent magnets, an R-T-B system rare earth permanent magnet has been adopted in various types of electric equipment for the reasons that its magnetic properties are excellent and that its main component Nd is abundant as a source and relatively inexpensive.
Research and development directed towards the improvement of the magnetic properties of the R-T-B system rare earth permanent magnet, more specifically, the improvement of a residual flux density, a coercive force and a maximum energy product, have mainly progressed. However, magnetizability has recently become a focus of attention in such research and development. To saturate a magnetization, the R-T-B system rare earth permanent magnet requires a magnetizing field higher than that of a ferrite magnet. For example, when a ring-formed R-T-B system rare earth permanent magnet is used as a rotator of a motor, there are some cases where the ring-formed R-T-B system rare earth permanent magnet is incorporated into the motor and a coil for a motor is then wound around the ring-formed R-T-B system rare earth permanent magnet for magnetization. When the motor is small, in order to obtain a certain number of windings, the diameter of a coil wire becomes thin, and thus, a high current cannot be passed through the coil. Accordingly, a sufficient magnetizing field cannot be applied to an R-T-B system rare earth permanent magnet. Therefore, an R-T-B system rare earth permanent magnet used for the above purpose is required to have as high as possible magnetizability in a low magnetizing field.
For example, Japanese Patent Laid-Open No. 2002-356701 discloses an R-T-B system rare earth permanent magnet with excellent magnetizability, which is a sintered body of rare earth alloy, the mean composition of the main phase of which is represented by (LR1-xHRx)2T14A (wherein T represents Fe or a mixture of Fe and at least one transition metal element other than Fe, A represents boron or a mixture of boron and carbon, LR represents at least one light rare earth element, HR represents at least one heavy rare earth element, and 0<x<1). This rare earth alloy sintered body comprises crystal grains having a plurality of at least one of the first main phase consisting of a composition represented by (LR1-pHRp)2T14A (0≦p<x) and the second main phase consisting of a composition represented by (LR1-qHRq)2T14A (0≦q<1).
Japanese Patent Laid-Open No. 2002-356701 discloses a technique for improving magnetizability without decreasing magnetic properties. However, a magnetizing field of approximately 0.8 MA/m (10 kOe) is required to obtain a magnetization rate of approximately 50%. Therefore, it is desired to obtain a magnetization rate of approximately 50% in a lower magnetizing field.
Moreover, Japanese Patent Laid-Open No. 2003-217918 discloses a rare earth sintered magnet, which comprises, for the purpose of improving magnetizability, 25% to 35% by weight of R (wherein R represents at least one rare earth element (providing that the rare earth elements include Y) and Nd occupies 50 or more atom % of R), 0.8% to 1.5% by weight of B, 8% or less by weight of M (at least one selected from a group consisting of Ti, Cr, Ga, Mn, Co, Ni, Cu, Zn, Nb and Al) as necessary, the balance T (Fe, or Fe and Co), and inevitable impurities. This rare earth sintered magnet has crystal structure in which an Fe phase comprising 80 or more at % of FeACo1-A remains at a size of 0.01 to 300 μm in the sintered body. The rare earth sintered magnet has a magnetization rate Br (0.2 MA/m)/Br (2.0 MA/m) of 59% or more, which is evaluated with a residual flux density, and a magnetization rate φ, (0.3 MA/m)/φ (4.0 MA/m), of 4% or more, which is evaluated with a flux.
However, the above values obtained in Japanese Patent Laid-Open No. 2003-217918, that is, a magnetization rate Br (0.2 MA/m)/Br (2.0 MA/m) of 59% or more, which is evaluated with a residual flux density, and a magnetization rate φ, (0.3 MA/m)/φ (4.0 MA/m), of 4% or more, are not considered to be good magnetizability.
According to the studies of the present inventors, with regard to an R-T-B system rare earth permanent magnet having a high magnetization rate in a low magnetic field, a magnetization characteristic curve indicating a fluctuation in a magnetization rate by a magnetizing field tends to gently incline. This is to say, a higher magnetizing field is required to reach a magnetization rate close to 100%, and it is not desired.